1. Field of the Invention This invention relates to oxide dispersion-strengthened anodes for use in fuel cells, in particular nickel-aluminum (Ni--Al) oxide dispersion-strengthened anodes for use in molten carbonate fuel cells. This invention also relates to a process for producing said oxide dispersion-strengthened anodes. Oxide dispersion-strengthened anodes produced in accordance with this process have improved anode creep resistance properties.
2. Description of Prior Art
Molten carbonate fuel cells generally comprise two electrodes with their current collectors, a cathode and an anode, an electrolyte tile making contact with both the electrodes, and a cell housing to physically retain the cell components and to provide contacts between the electrodes and the reactant gases. Under fuel cell operating conditions, in the range of about 500.degree. C. to about 700.degree. C., the entire electrolyte tile, consisting of the carbonate and the inert support material, forms a two phase structure with liquid carbonate and said inert support. The electrolyte diaphragms of this type are known as "matrix type" or "paste type" electrolytes. The electrolyte is in direct contact with the electrodes where the three-phase reactions (gas-electrolyte-electrode) take place. Hydrogen is consumed in the anode area producing water, carbon dioxide and electrons. The electrons flow to the cathode through an external circuit producing the desired current flow. At the anode there must be ready entry for the reactant gas, ready exit for the chemical reaction products and ready exit for the product electrons. To maintain a high level of stable, long term performance, both electrolyte and electrode design and properties must be optimized and stabilized at the gas-electrolyte-electrode interface.
Porous anodes of nickel, cobalt, or copper have been previously used in molten carbonate fuel cells. These anodes typically require stabilizing agents to maintain porosity and surface area during fuel cell operation. The stabilizing agents are usually added in about 1-10 weight percent, based upon the metal. The stabilizing particles are dispersed on the base metal surface, prohibiting the structure from sintering at molten carbonate fuel cell temperatures of 500.degree. C. to 700.degree. C.
Molten carbonate fuel cells have typically used nickel, cobalt, and copper based anode structures. These anodes tend to be dimensionally unstable, losing thickness by creep deformation within the fuel cell stack. Creep deformation occurs as a result of a holding force applied to keep the components in a fuel cell stack in good contact. Creep of electrodes occurs by combination of at least three different creep mechanisms: particle rearrangement, sintering, and dislocation movement. The surface dispersion of stabilizing particles used in prior art methods does not inhibit creep by dislocation movement. The creep of these anodes under the loaded conditions of a fuel cell stack is not acceptable.
Various methods have been used to attempt to inhibit creep deformation in the anode structures. One method has been to internally oxidize the alloying metal used in the base metal-alloying metal composition typically used to form the porous anode structures. For example, U.S. Pat. No. 4,999,155 teaches a method for forming a porous oxide dispersion-strengthened molten carbonate fuel cell anode in which a Ni--Al alloy powder is formed into a structure and internally oxidized by heat treating in the presence of a NiO Rhines pack. The NiO Rhines pack is a mixture of NiO and Ni and includes some inert particles, such as Al.sub.2 O.sub.3. U.S. Pat. No. 5,229,221 teaches a method for fabricating anodes in which an alloy powder having a base metal and an alloying metal is preformed into a desired shape and sintered in pure dry H.sub.2, a dry inert gas, or a vacuum, and subsequent thereto, oxidized in-situ in a fuel cell. U.S. Pat. No. 4,659,379 teaches a method for producing a nickel anode electrode in which a nickel alloying material is oxidized to produce a material whose exterior contains nickel oxide and whose interior contains nickel metal throughout which is dispersed the oxide of the alloying material. The oxidized material is then reduced and sintered to convert the nickel oxide outer layer to nickel metal and to provide an interior containing nickel metal throughout which is dispersed the oxide of the alloying material. Finally, U.S. Pat. No. 3,779,714 teaches a process for producing dispersion-strengthened hard metal by internal oxidation in which a powder alloy comprising a matrix metal and a solute metal and oxidant comprising an in-situ heat-reducible metal oxide and a finely divided hard refractory metal oxide are combined into an intimate mixture. The alloy mixed with the oxidant is internally oxidized by heating to oxidize the solute metal of the alloy and to form a residue of the oxidant. Thereafter, the internally oxidized alloy and the oxidant residue are thermally coalesced into dispersion-strengthened metal stock. Suitable matrix metals and solute metals for formation of the alloy are indicated to be nickel and aluminum, respectively, while a suitable heat-reducible metal oxide and a finely divided, hard refractory metal oxide for formation of the oxidant are indicated to be NiO and Al.sub.2 O.sub.3. Dispersion-strengthened hard metal stock produced in accordance with the teachings of the '714 patent is not suitable for use as an anode in a fuel cell, lacking the porosity required in such anodes.
A dispersion-strengthened nickel-base alloy comprising correlated amounts of iron, chromium, and aluminum is taught by U.S. Pat. No. 3,912,552. A process for producing an oxide dispersion-hardened sintered alloy based on a metal having a high melting point in which a powder mixture of the base metal and a dispersoid comprising a metal oxide powder is pressed into a pressed, blank form and sintered at temperatures such that the dispersoid is decomposed into its constituent components and the constituent components are homogeneously dispersed throughout the base metal is taught by U.S. Pat. No. 5,049,355. U.S. Pat. No. 5,110,541 teaches mixing an Al-based intermetallic compound with Ni to form a slurry which is shaped into a sheet or a tape and subsequently sintered to form a porous electrode.
U.S. Pat. No. 4,714,586 teaches a method for preparing stable Ni--Cr anodes for use in a molten carbonate fuel cell stack in which a low chromium-to-nickel alloy is provided and oxidized in a mildly oxidizing gas of sufficient oxidation potential to oxidize chromium in the alloy structure. A steam/H.sub.2 gas mixture in a ratio of about 100/1 and at a temperature below 800.degree. C. is used as the oxidizing medium.
In general, known processes for producing oxide dispersion-strengthened anodes have the disadvantages that they require multiple heating steps and employ atmospheres, such as atmospheres having very high H.sub.2 O/H.sub.2 ratios, which are fairly corrosive to metallic furnace walls containing such atmospheres. For example, the '586 patent describes the fabrication of a Ni--Cr oxide dispersion-strengthened anode by first sintering a Ni--Cr anode plaque and then internally oxidizing the anode in an atmosphere having a high H.sub.2 O/H.sub.2 ratio. The '379 patent requires multiple oxidation and sintering steps, the oxidation being carried out in air or in an H.sub.2 O/H.sub.2 atmosphere while sintering is performed in a reducing atmosphere. The '155 patent also teaches a process employing multiple heating steps, but more importantly, the pre-oxidized surface layer produced in accordance with the disclosed process is not cohesive with the internal volume of the resulting anode. Finally, in accordance with the teachings of the '122 patent, internal oxidation is allowed to take place in the fuel cell. The stringent requirement for the sintering atmosphere makes it difficult to carry out the process in a continuous sintering furnace, thereby impeding the production rate. In addition, in-situ internal oxidation is a process that changes the anode microstructure and, thus, may effect the electrochemical performance of the fuel cell.